1. Field of the Invention
This invention relates to pulsed laser source systems, and particularly relates to methods and apparatus for ultraviolet, pulsed, excimer laser source systems with high repetition rates. Such laser systems are useful as light sources in nanolithography systems for production of electronic devices.
2. Description of Related Art
The manufacture of modern electronic devices, commonly referred to as integrated circuits (ICs) or chips, requires a number of fabrication technologies. One of the most critical of such fabrication technologies is lithography, the process of patterning the billions of structures that form the individual components of the devices on the semiconductor wafers. Advances in the manufacture of electronic devices have required the patterning of ever smaller structures on the wafers, which, for the process of lithography, is referred to as requiring higher (i.e., finer) patterning resolution.
A key element in a lithography system that enables it to achieve a fine patterning resolution is its light source, which in modern lithography systems is an ultraviolet excimer laser due to its short wavelength. Typically, modern lithography systems use an Argon Fluoride (ArF) excimer laser source that emits radiation of 193 nanometer (nm) wavelength. Due to the fundamental physical operating mechanism of such a laser, it operates only as a pulsed source, with a typical pulse repetition rate of a few hundred to a few thousand pulses per second.
A modern lithography system with an excimer laser source also comprises a high-resolution, large-field projection lens that creates an image of a master pattern present on a mask onto the semiconductor wafer. The overall performance of the lithography system is determined by the projection lens, the light source, the mask, and several other factors. Current state-of-the-art lithography systems are capable of producing device structures in high volumes with a minimum feature size in the vicinity of 22-45 nm. With such small feature sizes, electronic chips with several billion transistors can be produced.
The demands on electronic systems to operate at ever greater speeds and have ever greater storage capacities are requiring more advanced chips with minimum feature sizes smaller than 22 nm. Modern lithography systems are incapable of patterning electronic structures with such small features with sufficiently high production throughputs for required cost efficiencies. There is thus a need to develop advanced lithography systems that can provide a patterning resolution significantly finer than 22 nm and patterning throughput of, for example, 100 or more wafers per hour. Such lithography systems are currently not available. To meet these objectives, many new lithography approaches are being investigated in the semiconductor industry and at research institutions, including extreme ultraviolet lithography, maskless lithography, immersion lithography, and other.
Of these new approaches, maskless lithography holds particularly strong promise due to its many advantages, including high resolution and elimination of the mask as a requirement in the lithography process. (That the latter is significant can be recognized by noting that the cost of the mask set for patterning the layers of a modern chip exceeds five million dollars.) Examples of methods and apparatus for maskless lithography are disclosed in U.S. Pat. No. 6,312,134, Seamless, Maskless Lithography System Using Spatial Light Modulator, 2001; U.S. Pat. No. 6,707,534, Maskless Conformable Lithography, 2004; U.S. Pat. No. 6,870,554, Maskless Lithography with Multiplexed Spatial Light Modulators, 2005; and U.S. Pat. No. 7,164,465, Maskless Lithography with Sub-Pixel Resolution, 2004.
In a maskless lithography system, the conventional hard mask as used in a typical optical projection lithography system is replaced by a spatial light modulator (SLM) array. Each element (i.e., individual element) in the SLM array can be programmed to be “On” or “Off”, i.e., reflective or nonreflective for a reflective-type SLM (or transmissive or nontransmissive for a transmissive-type SLM), so that the collection of all the beams emerging from an SLM array can be programmed to represent any desired pattern of light pixels that can then expose a photosensitive medium to create the corresponding pattern therein.
State-of-the-art SLMs have modulator elements of size in the vicinity of 10 micrometer×10 micrometer. In a maskless lithography system, by using a projection lens with a reduction ratio of approximately 200:1, an image pixel size of (10 micrometer)/(200)=50 nm can be produced. Thus, in order to improve the resolution of a maskless lithography system, the modulator element size must be reduced or the projection lens reduction ratio must be increased, both of which avenues are difficult.
It will therefore be beneficial to devise a technique that provides higher resolution for a maskless lithography system than the minimum feature size (“pixel size”) printed on the basis of the SLM element size and the projection lens reduction ratio.
Methods and apparatus for maskless lithography for providing a resolution finer than a pixel size, i.e., sub-pixel resolution, have been developed and are disclosed in U.S. Pat. No. 6,717,650, Maskless Lithography with Sub-Pixel Resolution, 2004, and U.S. Pat. No. 7,170,669, Spatial Modulator with Minimized Heat Absorption and Enhanced Resolution Features, 2007. These methods and apparatus define sub-pixel-size features by partial overlap between pixel-size features, exploit nonlinear photoresponse characteristics of the imaging media, and effectively use massively parallel bit addressing for full-pattern definition and high throughput.
In addition to the above considerations, the performance achievable by maskless lithography systems is dependent not only upon the ability of the SLM to rapidly transfer the pattern information from the data file to the imaging medium, but also upon the ability of the light source to illuminate the SLM with a new pulse every time the SLM frame (i.e., the array of all the modulator elements) is refreshed (i.e., provided a new set of pattern data). Modern SLMs can have frame refresh rates as high as 25 kHz, i.e., all the modulator elements can be provided with new “On” or “Off” information 25,000 times per second. In order to utilize such a high frame refresh rate capability, the light source must also be able to provide the same number of pulses per second. Modern excimer laser light sources are available with pulse repetition rates that are limited to approximately 6 kHz. Available light sources are therefore inadequate for implementation in maskless lithography systems with the highest refresh rate SLM arrays.
Therefore, there is a need to develop an ultraviolet excimer laser light source capable of providing pulses at repetition rates in the vicinity of 25 kHz and preferably even higher.
It is an object of this invention to provide a method for producing pulsed ultraviolet laser radiation at high repetition rates.
It is another object of this invention to provide an apparatus for producing pulsed ultraviolet excimer laser radiation with repetition rates of tens of thousands of pulses per second.
It is yet another object of this invention to provide a high-resolution maskless lithography method and apparatus utilizing a high-repetition-rate laser light source for illuminating a spatial light modulator array.
With the above examples of objects, other objects of this invention will be evident to those skilled in the art of semiconductor manufacturing, lithography, and related fields.
An advantage of the invention is that it enables effective utilization of high-refresh-rate spatial light modulators in maskless lithography systems for achieving high throughputs and high resolutions.
Another advantage of the invention is that it provides the ability to produce high-repetition-rate laser pulses using lower-repetition-rate laser sources.
Yet another advantage of this invention is that it enables the optimization of the combined operation of the illumination source and the spatial light modulator array in a maskless lithography system to achieve optimum throughput and resolution.
With the above examples of the advantages, other advantages of this invention will be evident to those skilled in the art of semiconductor manufacturing, lithography, and related fields.